24 Mechanisms of Photodegradation and Stabilization of Polyolefins GERALD SCOTT University of Aston in Birmingham, Gosta Green, Birmingham B4 7ET, England
Summary The most important initiation process involved in the early stages of the photo-oxidation of polyethylene is shown to be hydroperoxide photolysis associated with the decay of vinylidene groups. This is followed by carbonyl photolysis occurring primarily by the Norrish type II process. The most effective uv stabilisers belong to the preventive class of antioxidants. The nickel dialkyl dithiocarbamates which are important members of the peroxide decomposer class are particularly effective and are found to prevent the decay of vinyl idene and the formation of hydroperoxide both during processing and during subsequent uv irradiation. Carbonyl triplet and oxygen singlet quenching appear to play only a minor role with typical nickel complex stabilisers. However, the nickel oximes behave both as uv screening agents and as radical trapping agents in model compounds. Introduction The mechanisms of thermal antioxidant action have been extensively studied over the past thirty years and can be considered to be reasonably well under-stood. Several comprehensive reviews are available (1-3), and antioxi dants have been classified into two main types (4,5), namely preventive and chain-breaking. The former act
340
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341
Photodegradation and Stabilization of Polyolefins
by interfering with the initiation step of the autoxidation (1) a n d t h e l a t t e r w i t h t h e p r o p a g a t i o n s t e p τ ... . Initiator χ. + R H
R. + 0
v. X.
• (
A
\
Initiation . (homolysis)
R O
Propagation
.
(chain reaction) R0 -
+RH
2
, ( 2 )R O · ' 2 / Λ Ν Γ
Λ
v
(i)
N
XH+R.
^ *
2
(ii).
(ii)
R0 H+R 2
Stable ΚΑ ι , Molecule
ψ
Although photo-oxidation
_ . Termination
,... (ιιι) N
v
involves the same reaction
sequence, the relative importance of the three stages differs.
In g e n e r a l
much more energy i s
available f o rthe initiation step i n the f o r m of u v light.
T h i s i s capable of photolysing a v a r i e t y o f
weak bonds i n p o l y m e r s to f r e e r a d i c a l s o r of a c t ivating double bonds to triplet states w h i c h behave as diradicals.
T h e rate of initiation (i)i s much
higher i n the case of photo-oxidation
than i t i s i n
thermal oxidation andhence the rate of termination is also higher.
(iii)
Consequently the length of the kinetic
chain (ii) i s shorter andchain-breaking
antioxidants
a r e relatively l e s s effective although i t i s observed that this type of antioxidant frequently shows s y n e r g i s t i c behaviour with uv stabilisers. It h a s b e e n r e c o g n i s e d ,
t h e r e f o r e , that the m o s t
important uv s t a b i l i s e r s fall into the preventive c l a s s (6) and t h r e e d i s t i n c t m e c h a n i s t i c uv s t a b i l i s e r s have been proposed.
types of
In c h r o n o l o g i c a l
o r d e r t h e s e a r e (a) u v a b s o r b e r s (6-8)
(b) p e r o x i d e
d e c o m p o s e r s (5,6,9-12) a n d (c) t r i p l e t q u e n c h e r s (13-20). (a)
UV
Absorbers
T h e main classes of compound contained are the
i nthis class
2 - h y d r o x y b e n z o p h e n o n e s (I) a n d t h e 2 - h y d r o x y -
benzotriazoles (II).
342
UV
L I G H T INDUCED REACTIONS IN
Ν
HO'
R
R I
POLYMERS
II
T h e s e w e r e believed originally to function entirely as u v s c r e e n i n g a g e n t s ( 6 ) . It w a s t h o u g h t t h a t , l i k e pigments such as carFon black o r titanium dioxide, they had the a b i l i t y to a b s o r b the uv light and d i s s i p a t e t h i s as t h e r m a l rather than as c h e m i c a l energy. Subsequent w o r k has shown that this c l a s s shows s o m e a b i l i t y to behave by the chain-breaking mechan i s m (11,21,22) and a l s o to quench t r i p l e t states (16,20). H o w e v e r , t h e r e i s l i t t l e doubt that uv s c r e e n ing i s one, and p r o b a b l y the m o s t i m p o r t a n t , of t h e i r functions s o that t h e y prevent o r reduce the rate of f o r m a t i o n of initiating f r e e r a d i c a l s i n the polymer. (b)
Peroxide decomposers
S o m e of the most powerful uv s t a b i l i s e r s belong to the c l a s s of peroxide decomposing preventive antioxidants and i thas been suggested that the m e c h a n i s m of this t y p e of u v s t a b i l i s e r i s not d i s t i n g u i s h a b l e f r o m t h e i r behaviour as t h e r m a l antioxidants although a l l peroxide d e c o m p o s e r s do not b e h a v e a s u v s t a b i l i s e r s (11). O f paramount importance i n the peroxide d e c o m p o s e r u v s t a b i l i s e r c l a s s a r e t h e m e t a l d i t h i o c a r b a m a t e s (III) (9,22,23,24) the dithiophosphates (IV) (11,24) and the c y c l i c phosphate e s t e r s (V) (11,25,26) w h i c h unlike m o s t antioxidants and uv s t a b i l i s e r s have v e r y high a c t i v i t y as both t h e r m a l and uv s t a b i l i s e r s . D e t a i l e d studies
III
IV
SCOTT
Photodegradation and Stabilization of Polyolefins
V i n m o d e l s y s t e m s h a v e s h o w n t h a t III - V g i v e r i s e to v e r y powerful L e w i s acid catalysts f o r the d e c o m p o s i t i o n of hydroperoxides i n a p s e u d o - f i r s t o r d e r d e c o m p o s i t i o n of hydroperoxide (27), i n d i c a t i n g t h a t t h e L e w i s a c i d i s n o t d e s t r o y e d "by o x i d a t i o n but p e r s i s t s i n the p o l y m e r u n t i l i t i s d e a c t i v a t e d o r lost by volatilisation. (c)
Excited State Quenchers
T h e suggestion that s o m e uv s t a b i l i s e r s m a y function by deactivating photo-excited states of m o l e c u l e s , p a r t i c u l a r l y of t r i p l e t carbonyl and singlet oxygen, a r o s e l a r g e l y a s a r e s u l t of f u n d a m e n t a l p h o t o c h e m i c a l studies in model s y s t e m s and many attempts have been made to invoke t h i s m e c h a n i s m to explain the activity of a v a r i e t y of p h o t o - s t a b i l i s e r s ( 1 3 - 2 0 ) . M a n y e f f e c t ive uv s t a b i l i s e r s a r e indeed efficient quenchers of p h o t o - e x c i t e d s t a t e s ( 2 0 ) , but s o a r e m a n y m o r e w h i c h a r e not and i n s o m e c a s e s j n o t a b l y t h e t r a n s i t i o n m e t a l a c e t y l a c e t o n a t e s ( V I ) , (28) q u e n c h e r s of t r i p l e t c a r b o n y l a r e among the most powerful photo-activators known (22).
VI T h e p r e s e n c e of p h o t o - a c t i v a t o r s i n
polymers
The failure to demonstrate a satisfactory r e l a t i o n ship between carbonyl triplet o r oxygen singlet
343
UV L I G H T INDUCED REACTIONS I N P O L Y M E R S
344
quenching ability and uv stabilising effectiveness necessitates a m o r e basic approach to the question of m e c h a n i s m and i n p a r t i c u l a r it i s important to understand the relative i m p o r t a n c e of photoactivators i n the various stages of t h e r m a l and environmental history of p o l y m e r s . The polyolefins a r e particularly suitable subjects f o r study s i n c e f o r m a l l y they s h o u l d not a b s o r b i n the uv above 285 n m which i s the effective cut-off point f o r uv light coming through the earth's atmosphere. In p r a c t i c e t h e y d o a b s o r b a n d i t h a s been a matter of conjecture since polyethylene w a s f i r s t d i s c o v e r e d a s to w h i c h of the v e r y m i n o r c h r o m o p h o r i c constituents of c o m m e r c i a l polyolefins are responsible for their photo-sensitisation. T h e possible stages in the history of a typical p o l y m e r during which s e n s i t i s e r s m a y be introduced a r e s h o w n i n T a b l e 1. Transition metal ions T h e r e i s c o n s i d e r a b l e evidence that s o m e t r a n s i t i o n m e t a l ions a r e powerful a c t i v a t o r s f o r both t h e r m a l and uv i n i t i a t e d oxidation of p o l y m e r s . The most effective of these a r e cobalt, i r o n , copper and m a n g a n e s e but t h e s e a r e not n o r m a l l y found a s catalyst residues from ionic polymerisation. T i t a n i u m , the m o s t l i k e l y c o n t a m i n a n t , i s not a v e r y powerful pro-oxidant. Nevertheless, its presence in s m a l l amount m a y partly account f o r the much l o w e r uv r e s i s t a n c e of H D P E a s c o m p a r e d with LDPE. The common transition metal ions a r e , however, likely contaminants during processing and particularly reprocessing operations when contaminants (from biological o r other s o u r c e s ) a r e not adequately r e m o v e d f r o m w a s t e p l a s t i c s . The effects of typical pro-oxidant transition metal ion complexes on the uv stability of polyethylene and p o l y p r o p y l e n e a r e s h o w n i n T a b l e 2 . It i s c l e a r that cobalt a n d i r o n i n soluble f o r m have a c a t a s t r o p h i c effect on the uv s t a b i l i t y of both p o l y m e r s . M o r e o v e r , stability i s affected in proportion to the
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345
Table 1
Possible photosensitisers
Stage 1
Polymer
manufacture
2
P r o c e s s i n g and fabrication
3
Environmental exposure
Unsaturation, Transition metal ions (hydroperoxide, carbonyl compounds by adventitious oxidation)
Hydroperoxides, carbonyl compounds (by high t e m p e r a t u r e oxidation, l i m i t e d oxygen supply) Transition metal ions (from machinery or compounding ingredients)
Polycyclic hydrocarbons (atmospheric pollution) C a r b o n y l (by photolysis of h y d r o p e r o x i d e s ) Unsaturation (by photolysis of ketones) Singlet oxygen and derived hydroperoxides (by q u e n c h ing of c a r b o n y l t r i p l e t etc with triplet oxygen) Transition metal ions ( p a r t i c u l a r l y i r o n and copper)
346
Table
UV L I G H T INDUCED REACTIONS I N P O L Y M E R S
2
E f f e c t of t r a n s i t i o n m e t a l acetylacetonates on the oxidation of L D P E
during p r o c e s s i n g and on U V
Additive
Concentration m o i s / 100 g
Fe(acac),.
2 x 1 0 -3 1 χ 10 0 . 7 χ 10
Co(acac)
2 x 1 0
c
-5
-3
1 . 4 χ 10 -3
irradiation
Carbonyl index (before exposure)
T i m e to embrittle ment (hrs)
10.22
120
4.64
216
1 .10
260
43.57
0
22.82
50
0.51
1968
0.64
600
-4 1 . 7 χ 10
2.70
510
Ce(acac)^
-3 2 χ 10
0.48
700
Zn(acac)
-3 1 χ 10
0.38
1370
0.15
2100
Ni(acac)
2 x 1 0
2
Mn(acac) .2H
